Allow installing along with 0.11 phpstan

Travis build and composer.json php requirement updated to follow
official phpstan extensions

Author: Karolis Daužickas

.travis.yml

@@ -4,7 +4,7 @@ language: php
 php:
   - "7.1"
   - "7.2"
-  - nightly
+  - "7.3"
 
 env:
   - PREFER_LOWEST=no

composer.json

@@ -4,8 +4,9 @@
     "keywords": ["enum", "phpstan"],
     "license": "MIT",
     "require": {
+        "php": "~7.1",
         "myclabs/php-enum": "^1.2",
-        "phpstan/phpstan": "^0.10"
+        "phpstan/phpstan": "^0.10|^0.11"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.0"